Mobile network base station controller comprising a central gateway and a physically separate remote gateway which communicate via a remote link

ABSTRACT

A remote mobile network part ( 2, 11, 12 ) may be on an aircraft, a ship, or in a geographically remote location. It comprises a gateway ( 6 ) which communicates with a central gateway ( 9 ) via a satellite link. The two gateways ( 6, 9 ) together are a base station controller (BSC), the satellite link being used for BSC-internal communication. Such communication can be optimised to make best use of the limited bandwidth available on the satellite link.

This is a CONTINUATION of PCT/IE2004/000114 filed 2 Sep. 2004 andpublished in English.

INTRODUCTION

1. Field of the Invention

The invention relates to communication involving persons at a remotelocation such as on a movable vehicle or craft, either air-borne,water-borne, or land-borne, or in a remote geographical location withfew telecommunication resources.

2. Prior Art Discussion

It is known to provide a mobile network cell in a craft for use bypersons who are travelling. Communication between the cell and a fixedmobile or land-line network uses an existing communication link for thecraft. An example is the existing satellite link of an aircraft.However, a problem with this approach is that there is often verylimited bandwidth available on this link, and so communication isunreliable and significant compression is required. Such links canintroduce a large latency, for example a 5 second delay for a singlecall set-up packet.

It is also known to provide a mobile network infrastructure in remotelocations, communication with the remainder of the network being via asatellite link. However, similar problems to those set out above apply.

The invention is therefore directed towards providing a communicationsystem and method for more effective use of existing links betweenremote locations and major network infrastructure.

SUMMARY OF INVENTION

According to the invention, there is provided a mobile network basestation controller comprising physically separate remote and centralgateways, in which the remote gateway communicates with cell equipmentfor a remote mobile network cell, the central gateway communicates witha mobile network node, and the gateways perform internal base stationcontroller communication using a remote communication link.

In one embodiment, the remote communication link is a satellite link.

In one embodiment, the remote communication link is a craft-to-groundcommunication link.

In one embodiment, the remote communication link is terrestrial basedradio, microwave, cable or optical fibre.

In one embodiment, at least one gateway terminates some signallingreceived from an external entity to minimise internal traffic on theremote communication link.

In one embodiment, the remote gateway terminates heartbeat signallingfrom remote network nodes.

In another embodiment, the remote gateway terminates radio measurementsignalling.

In one embodiment, the remote gateway terminates link measurementsignalling.

In one embodiment, the central gateway terminates link managementsignalling.

In one embodiment, the central gateway terminates heartbeat signallingfrom a mobile network node.

In one embodiment, each gateway dynamically chooses an appropriatecarrier on the remote link according to nature of the signals tooptimise bandwidth usage.

In one embodiment, each gateway dynamically switches between use of dataand voice channels of the remote link.

In one embodiment, each gateway dynamically maps external interface datato lower-bandwidth data for transmission on the remote link.

In a further embodiment, the remote gateway extracts a dialled numberfrom multiple messages, and transmits only the dialled number data fromsaid multiple messages.

In one embodiment, the remote gateway adds additional data to thedialled number data, and the central gateway interprets the additionaldata to generate mobile network signals for the mobile network node.

In one. embodiment, the remote gateway uses an ISDN user-to-usersignalling field to transfer the additional data.

In one embodiment, the remote gateway uses DTMF to transmit additionaldata, and the DTMF is terminated at the central gateway.

In one embodiment, the remote gateway batches location update requestsaccording to the nature of the remote link.

In one embodiment, the remote gateway batches text or multi-mediamessages according to the nature of the remote link.

In one embodiment, authentication related information exchanged betweenthe central gateway and a network node is batched for transmission toand from the remote gateway which handles authentication of individualmobile subscribers.

In one embodiment, the gateways use compression for at least someinternal communication on the remote link.

In one embodiment, each gateway buffers speech packets and combines aplurality of buffered speech packets into a single transmission packet.

In a further embodiment, the transmission packet is an RTP packet.

In one embodiment, the gateways perform transcoding and rate adaptationat the most appropriate gateway according to the nature of the remotelink.

In one embodiment, the central gateway comprises an operations andmaintenance function for both gateways.

In one embodiment, the remote gateway comprises an operations andmaintenance function for itself and the remote mobile network cell.

In one embodiment, the operations and maintenance function downloadsupdates to the remote gateway.

In one embodiment, the operations and maintenance function uploads faultinformation, status information and statistics from the remote gateway

In one embodiment, the central gateway maintains a database of datapertaining to remote gateways with which it communicates.

In one embodiment, the remote gateway maintains a database of registeredsubscribers for the remote cell.

In one embodiment, the remote gateway is linked to an entertainment orpublic announcement system.

In one embodiment, the remote gateway allows calls to be interrupted bythe public announcement or entertainment system.

In one embodiment, the remote gateway determines if a locally-initiatedcall or message is to a person located in its cell, and avoids use ofthe remote link if this is the case.

In one embodiment, the remote gateway is configurable to handle textmessage traffic only

In one embodiment, the remote gateway rejects outgoing voice and datacalls from subscribers registered in the remote mobile network cell whenthe system is operating in text message only mode.

In one embodiment, the central gateway sends a signal to the HLR of aregistered subscriber to divert incoming calls when the system isoperating in text message only mode

In one embodiment, the central gateway detects Ocean Region changes froma remote gateway logon message and from the full address of the remotecraft, builds a full correct dialled number including the Ocean regionthat will be used to terminate voice calls to each subscriber

In a further embodiment, at least one of the gateways automaticallygenerates a response to a network node without using the remote link.

In one embodiment, the gateway automatically generates the responsewithin a timeout period set by the network node to which it isresponding.

In one embodiment, he gateway is pre-configured with rules toautomatically generate said responses.

In one embodiment, said gateway is the central gateway.

In one embodiment, the central gateway automatically generates aresponse to an MSC class mark enquiry.

The invention also provides a mobile network cell comprising a basestation controller as defined above, and a base station transceiver.

The invention also provides a method of communication between a remotemobile network cell and a central mobile network node, the methodcomprising a base station transceiver of the remote cell communicatingwith a remote gateway of a base station controller as defined above inany embodiment, the remote gateway processing signals received from thetransceiver and transmitting internal base station controller signals tothe central gateway via a remote link.

The invention also provides a mobile station comprising an adapter forswitching between a normal mode and an in-craft mode.

In one embodiment, said mobile station uses less output power for thein-craft mode.

In one embodiment, said mobile station automatically restricts operationto an in-craft cell when in the in-craft mode.

In one embodiment, the mobile station accepts over-the-air setting ofring and alert tones to silent mode.

In one embodiment, the adapter has an externally visible indicatorindicating mode status.

DETAILED DESCRIPTION OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the followingdescription of some embodiments thereof, given by way of example onlywith reference to the accompanying drawings in which:

FIG. 1 is a block diagram of a communications system of the invention;and

FIGS. 2 and 3 are block diagrams of remote and central gateways of thesystem.

DESCRIPTION OF THE EMBODIMENTS

Referring to FIG. 1 a communication system 1 of the invention comprisesa remote part 2 and a ground part 3. In this embodiment the remote partis for an aircraft mobile network cell, however, in other embodiments itmay be for a geographically remote location such as a remote village, ora ship for example. The remote part 2 comprises a conventional BTS 5linked by a GSM A-bis link to a gateway 6, called the “remote gateway”.The remote gateway 6 uses conventional aircraft satellite communicationequipment (AES) 7 and corresponding ground equipment (GES) 8 tocommunicate with a central gateway 9 on the ground. The gateway 9 inturn communicates with a conventional Mobile Services Switching Centre(MSC) 10. The MSC 10 may already exist as part of an operator's PublicLand Mobile Network (PLMN). The MSC 10 is linked with other nodes of itsPLMN in conventional manner.

The gateways 6 and 9 are together a BSC for the remote cell of themobile network (in the aircraft in this case). Thus the BSC is splitinto two physically separate parts, namely the gateways 6 and 9. Thegateways 6 and 9 use the air-to-ground link to communicate, and optimumuse is made of the available bandwidth by mapping the standard A andA-Bis protocols onto an optimised internal lightweight protocol betweenthe gateways. More flexibility is possible to minimise trafficinternally within the logical BSC than would be possible in the A-bisand A links on either side of the BSC.

The following describes an embodiment in which the network is GSM andthe node which is split is a BSC. However, if the network were 3G the“split” note would be an RNC, of if the network were GPRS the split nodewould be a BSC/PCU.

Conventional mobile stations 11 and 12 communicate with the BTS 5 eitherusing a wired RF connection or a conventional wireless connection.

The fact that the BSC is split between the two gateways means that thecentral-to-remote communication can be of very low bandwidth and in aprotocol tailored for the particular circumstances. The internal trafficis minimised and optimised by the following techniques:

-   -   (a) Terminating non-essential received signalling at each        gateway 6 and 9, avoiding it being routed on between the        gateways, or generating an automatic response without using the        link. One example is the central gateway 9 receiving from an MSC        a class mark enquiry to determine handset capabilities and voice        coding capabilities. The central gateway 9 automatically        generates a response without using the link to communicate with        the remote gateway 6. The “intelligence” to generate the        automatic response arises from configuration settings of the        central gateway 9. This response is transmitted within the MSC's        timeout period, so that its requirements are met. Another        example is at the remote site, in which a BTS and its        neighbours' signal strength data is measured and reported by the        mobile station for handover decision making by the BSC. Because        the remote site is confined in nature and handover may not be        necessary, there is no need for relaying such data, and it is        simply terminated. The BTS does not expect a response, and there        is thus no timeout.    -   (b) Compression of voice, data, and signalling streams. In one        embodiment, RTP speech packets are buffered in the gateways and        combined in a single RTP packet to reduce the packet overhead.        For example, in GSM, speech packets are sent every 20 ms. If        these packets were sent in individual RTP packets the RTP header        would be almost the same size as the speech payload. By        combining several GSM packets in each RTP packet the payload to        packet header ratio can be significantly improved. The delay        incurred by this combining process is negligible especially when        using Low Earth Orbit satellites. Alternatively, conventional        RTP header compression can be used.    -   (c) Performing GSM transcoding and rate adaptation at the most        appropriate point (remote gateway 6 or central gateway 9)        depending on the craft-to-ground link used.    -   (d) Batching of certain non-real time messages such as some        location update requests and SMS messages.    -   (e) Use of a local operations and maintenance (O & M) function        on the remote gateway 6 for configuration of the BTS 5.    -   (f) Optimising the use of protocols such as X.25 by reducing the        packet size (in the Aero-H environment this forces the MES to        use an R channel instead of a T channel which improves        performance)    -   (g) Terminating certain signalling sequences at the gateway and        transmitting the essential data over the satellite link using a        different protocol. For example, on A-bis, sending an SMS        message involves a first A-bis message containing the text        message followed by 2 acknowledge messages. This procedure is        executed by the gateway from the BTS point of view, however, the        text message is extracted and sent across the satellite as a        single X.25 packet pair (packet, packet-acknowledge). In        essence, each gateway maps the external data-rich messages to a        low-bandwidth set of messages containing only the essential        data. A simple example is extraction of the dialled number from        several messages on the BTS-BSC Abis interface, and transmitting        only the dialled number without the ancillary data on the        satellite link. Another example is extraction of essential        encryption data from the BTS-BSC A-bis interface and        transmitting it in a data channel of the satellite link.    -   (h) Dynamically choosing satellite channels according to the        required bandwidth. For example a single Aero-H channel can be        used when one voice channel is required, and if several voice        calls are presented at the same time a higher capacity channel        (e.g. Swift 64) can be set-up to handle these calls in a more        cost effective way.    -   (i) Dynamically choosing a protocol for the satellite        air-to-ground link, the protocol chosen depending on the type of        traffic. This feature is described below.        Dynamic Protocol Selection (i)

Most satellite systems are capable of supporting several differentphysical interfaces and protocols. For example the Inmarsat Aero-Hsystem supports ISDN based switched circuits and X.25 packet dataprotocol. The ISDN-based circuits and their associated signallingchannels are usually used to setup and carry voice calls. The X.25channel is used to carry bursts of data. The gateway will use X.25 forlocation updating (registration) and SMS traffic. Once the mobile hasbeen successfully registered on the system it can then make calls whichare routed over the ISDN channel.

Setting up these voice calls involves the gateway 6 decoding the Abismessages and mapping the contents of these messages onto the ISDNsignalling channel. This ISDN signalling channel does not support therich set of features that Abis supports and so additional techniques areemployed to carry these essential signals across the ISDN channel. Theremote gateway 6 uses the following techniques to achieve this:

-   -   Adding extra digits to the dialled number, these extra digits        are interpreted by the central gateway 9 and used to generate        the corresponding signals on the A interface.    -   Use the ISDN “user-to-user” signalling field to transfer the        additional data.    -   Dynamically switching the circuit between “data” and “voice”        call, the call could be set up in data mode and when signalling        is complete switched to voice mode.    -   Use DTMF to transfer additional data, the DTMF is terminated and        interpreted by the central gateway 9.    -   Use a satellite sub-channel to transfer additional data.

Referring to FIG. 2 the remote gateway 6 comprises:

-   -   20: A BSC function which handles a subset of standard BSC        functionality including signalling adaptation and switching.    -   21: A speech and data transcoding and compression unit which        handles speech channel compression, decompression, transcoding        and rate adaption.    -   22: An O&M function which handles O&M operations for the BTS 5        and the remote gateway 6 itself.    -   23: A local maintenance terminal    -   24: A Local Control Panel (LCP)    -   25: A Subscriber Database

Referring to FIG. 3 the central gateway 9 comprises:

-   -   30: A BSC function which handles a subset of standard BSC        functionality including signalling adaptation and switching.    -   31: An O&M function handles O&M operations for the gateway 9,        associated mobile gateways 6 and BTSs.    -   32: A remote gateway 6 database which is used by the gateway 9        to determine the location of BTSs in terms of communications.    -   33: An O&M application 33 specifically dedicated to the        management of the central gateway, remote gateways and BTSs.    -   34: A local maintenance terminal (LMT).    -   35: A speech and data transcoding and compression unit which        handles speech channel compression, decompression, transcoding        and rate adaption.

As the satellite links used between aircraft and ground station aretypically low bandwidth and high cost, all unnecessary signalling isdiscarded or terminated by the gateways 6 and 9. For example, handoversmay not be necessary in some applications, so measurement reports thatare sent from the BTS 5 to the remote gateway 6 could be ignored. Othersignalling messages such as heartbeat and certain O&M messages may beterminated and handled by the gateway 6 or 9 in the same way that theBSC would handle these messages.

As for speech and signalling it is necessary to minimise O&M traffic onthe satellite link. The system can be configured so that only urgentfault information (alarms) will be forwarded immediately to the groundstation. Other non-urgent fault indications will be logged in the remotegateway 6 which may generate an immediate alarm to the ground station ifthe arrival rate of these minor fault indications exceeds a threshold.The alarm severity and threshold values associated with this behaviourare configurable.

For aircraft applications, the craft-to-ground link is based oncommercial satellite systems such as Inmarsat, Intelsat or terrestrialsystems such as NATS or microwave. The different satellite systems varygreatly in bandwidth and quality. The in-craft satellite modules(Aircraft Earth Station or Mobile Earth Station) expose many differentinterfaces such as ARINC 746 used in aeronautical applications andconventional IP or Frame Relay used principally in the maritimeindustry.

The gateways 6 and 9 are able to handle these different interfaces andsystems both in terms of the physical interfaces and protocols.Furthermore, the functional partition between the gateways 6 and 9 canbe configured to optimise the use of the satellite link. GSM transcodingand rate adaptation (TRAU) can thus be performed either on board thecraft by the gateway 6 or on the ground by gateway 9.

For example, in the case of Inmarsat Aero-H, transcoding is performed onthe craft by gateway 6. This is because the Aero-H module which supportsa CEPT E1 PCM link, compresses voice signals itself before sending overthe satellite link.

In a typical maritime example, a transparent IP link is availablebetween the gateways 6 and 9. In this scenario, the GSM encoded TRAUframes could be carried on the IP link using a streaming protocol suchas RTP. Alternatively, if the GSM encoding is not efficient enough forthe satellite link in question, transcoding can be performed by thegateway 6 which then uses a more efficient compression algorithm beforetransmitting to the satellite module.

The local maintenance terminal 23 of the remote gateway 6 supportsfunctions used by field personnel including, but not limited to thefollowing:

-   -   Read fault information    -   Read log files    -   Download statistics    -   Configure the BTS and gateway 6.    -   Upload new software versions to the BTS and gateway 6.    -   Test that the BTS and gateway 6 are operating correctly.

The LMT 23 is not necessary for normal operation of the BTS 5 or thegateways 6/9 which can be maintained remotely from the ground-based OMC.

The Local Control Panel 24 allows the crew to enable and disable thesystem and set the service. The Local Control Panel 24 also gives thecrew system status and fault indications.

Signalling from the BTS 5 is carried in an optimised format over thesatellite link. The BSC function in the central gateway 9 adapts thissignalling to standard “A” interface format before sending to the MSC.The BSC function terminates unnecessary traffic from the MSC andsimulates traffic from the BTS such as heartbeats that the MSC needs toreceive.

To establish a connection to a remote gateway 6, the central gateway 9must know the location and environment of the remote gateway 6 so thatan appropriate communication channel can be established. Thisinformation is maintained in the database 32. This maintains a log ofwhich remote gateways 6 are currently active and logged onto the system.This database also maintains a list of mobiles that are attached to eachof its BTSs 5. This is to allow implicit deregistration of these mobileswhen the BTS 5 and/or gateway 6 become unreachable. The subset of thedatabase 32 that contains information pertaining to a particular gateway6 is also stored in the subscriber database 25 on that gateway 6. Thisallows the gateway 6 to take subscriber data such as class of serviceinto account when processing calls on the craft.

The O&M function 31 in the gateway 9 is responsible for the managementof the gateway 9, the gateway 6, and the BTSs. It handles O&M data(faults, statistics, and configuration) exchanged with the airborne BTSand gateway 6 over the satellite link. The OMC uploads software andconfiguration data to the gateway 6 to allow on-line upgrades to the BTS5 and the gateway 6. The O&M function 31 communicates with aconventional OMC 40 using SNMP and with the gateway OMC 33 using anappropriate protocol.

The following describes the operation of the communication system of theinvention in more detail. It can be split into eight broad areas, namely

-   -   BTS and Gateway Logon    -   BTS and Gateway Logoff    -   Location Update    -   IMSI Detach    -   Mobile Originated Voice Call    -   Mobile Termninated Voice Call    -   Mobile Originated SMS    -   Mobile Terminated SMS

Taking each in turn

BTS and Gateway Logon

Once the craft has reached an appropriate location, the in-craft systemis started either automatically or manually by the crew. The gateway 6checks that the craft-to-ground module is already logged on to theground earth station and then sends a logon message to the gateway 9containing its identity and authentication information.

The gateway 6 communicates with other on-board systems to determineoperational parameters such as the location, altitude and state of thecraft. This equipment includes but is not limited to the following:

-   -   ARINC 429 bus or ARINC 746 ECL for location and altitude    -   Various discrete signals for Weight On Wheels status, seat-belt        status and door open status.    -   GPS system for craft location.    -   The Local Control Panel

The gateway 9 updates the database 32 with the gateway 6 identity anddata relevant to the communication channel. Contrary to the land basedGSM network where BTS sites are “wired” to a particular BSC, in theinvention different dynamically allocated communication channels may beused to connect a gateway 6 to a gateway 9. The gateway 9 must thereforekeep track of the gateway's 6 location and communications environment.

The gateway 9 maintains associations between the craft's identity (forexample an aircraft's AES id.) and location areas (LA) known to the MSC10 in the database 32. The MSC 10 will page subscribers in a particularLocation Area and the gateway 9 will use this association and itsknowledge of subscribers registered on each remote gateway to locate thegateway 6 on the craft to forward the paging message to the correct BTS5.

For example, Inmarsat uses four geostationary satellites to achieve nearglobal coverage, only the Polar Regions not being covered. Tocommunicate with an aircraft, the gateway 9 must know which of the fourocean regions the aircraft is flying in and therefore which satellite touse. The gateway 6 polls the SatCom module regularly to see if thesatellite receiver has logged onto a new satellite. This happens when anaircraft hands over from one satellite to another. If the Ocean Regionhas changed, the gateway 6 initiates a new logon procedure (in this casethe BTS is kept on-line so that mobiles are not required to re-register)and the gateway 9 updates the database 32 with the new area information.Alternatively, the gateway 9 may recognise that the craft has changedsatellite and establish a link through the new satellite without theneed for a logon procedure.

The gateway 9 will detect Ocean Region changes from the logon message.From the full address (e.g. X.25, E.164, Inmarsat number, IP, etc.) ofthe remote craft it builds the full correct dialled number (includingthe ocean region) that will be used to terminate voice calls to eachsubscriber. The dialled number is a combination of the ocean region,craft number and seat number. Each subscriber will be allocated a numberfor the flight duration. When the gateway 6 detects an incoming call ona particular number it will match that number to a subscriber toterminate the voice call.

BTS and Gateway Logoff

The gateway 6 may be shut down either automatically (e.g. when the craftapproaches a specific geographical location) or manually by the crew.When this happens the gateway 6 sends a logoff message to the gateway 9which updates the database 32 and sends a detach message to the MSC forall subscribers that were registered with the BTS.

Location Update

When a mobile on the craft loses signal from the ground PLMN it willscan for a new network and find the in-craft BTS's Broadcast ControlChannel (BCCH). Once locked to this new BCCH the mobile recognises thatit has changed location area (and PLMN) and so it will perform alocation update procedure.

In another embodiment, specially customised mobile stations 12 willstart to look for an “aircraft” network as soon as they are turned on in“aircraft” mode.

The mobile uses the Random Access Channel (RACH) to gain access to theBTS 5 and request a signalling channel.

The BTS forwards the request to the gateway 6 which responds with achannel assignment command to the BTS. The BTS in turn instructs themobile to switch to the allocated channel.

Once the mobile has switched to the signalling channel, it sends alocation update request to the BTS which forwards the request to thegateway 6. The gateway 6 sets up a connection to the gateway 9 (if oneis not already established) and uses this connection to forward therequest to the MSC.

The MSC responds by sending an authentication request to the mobilewhich is again relayed to the BTS and mobile across the gateways anddata connection.

The mobile device processes the authentication request and sends theresponse back to the MSC. The MSC checks the response and if it issuccessful, updates the VLR and HLR.

The MSC then instructs the gateway 9 (BSC) to clear the call. Thegateway 6 sends the clear command to the mobile. Once acknowledged bythe mobile, the gateways 6 can tear down the data connection.

In another embodiment, Location Update requests from mobile devices areimmediately acknowledged by the gateway 6. These requests are thenbuffered in the gateway 6 until a threshold of outstanding requests isexceeded or a timer expires, the requests are then batched and sent tothe gateway 9. The gateway 9 then sends location update requests to theHLR and obtains authentication triplets from the HLR for each of themobile stations. The triplets are batched and sent to the gateway 6which will use these triplets to authenticate the mobile stations beforethey are allowed to initiate or receive service requests. This mechanismmakes most efficient use of the signalling channel while ensuring thatall mobile stations are correctly authenticated.

If, for whatever reason, the location update and/or authenticationprocedures fail, the registration request from the mobile will still beacknowledged. This ensures that the mobile remains camped on theon-board cell and its power can be controlled by the gateway 6. Servicerequests to and from these mobiles will be refused by the system untilthe location update and authentication procedures have been completedsuccessfully.

Known subscribers may be included in different classes. The system maybe configured to restrict access and service to certain classes ofsubscribers. For example, in a given configuration first and businessclass subscribers may be allowed to make and receive voice calls whereaseconomy passengers may only be allowed to send and receive SMS messages.

IMSI Detach

The IMSI detach procedure is initiated when the mobile is switched off.A detach message is sent over the satellite link to the ground basedMSC. The MSC updates the subscriber's HLR so that new calls to thesubscriber will be terminated in the home PLMN (typically diverted tovoice mail) and will not use satellite resources unnecessarily.

If the ground station loses contact with the airborne gateway it willinitiate an automatic detach procedure (implicit deregistration) after aconfigurable time for all the subscribers that had been logged on to thecorresponding BTS.

Mobile Originated Voice Call

Once the mobile has registered successfully on the network it can makean outgoing call. The air interface used is standard GSM so the mobilesees no difference between the airborne BTS and a normal land BTS.

The mobile uses the Random Access Channel (RACH) to gain access to theBTS and request a signalling channel.

The BTS forwards the request to the gateway 6 which responds with achannel activation command to the BTS, the BTS activates a signallingchannel (SDCCH). The gateway 6 now sends an Immediate Assignment Commandto the BTS which is forwarded to the mobile on the Access Grant Channel(AGCH). The mobile switches to the SDCCH and sends a CM Service Requestfor a Mobile Originating call. The gateway 6 replies with an acceptmessage and the mobile sends a Setup message which contains the dialleddigits.

The gateway 6 uses these digits to build and send an ARINC 746 Setupmessage to the SatCom/NATS module. This Setup message is routed throughthe satellite network to the GES on the ground. The GES forwards theSetup request to the gateway 9, the gateway 9 uses the contents of theSetup message to build and send a standard A interface setup message tothe MSC.

The MSC routes the call to the called subscriber.

The gateway 6 now instructs the BTS to activate a traffic channel (TCH).The gateway 6 sends an Immediate Assignment command through the BTS tothe mobile which switches to the traffic channel. The gateway 6instructs the BTS to release the signalling channel (SDCCH).

Once the called subscriber has been located by the PSTN, an alertingmessage is sent by the PSTN through the MSC, gateway 9, GES, andSatCom/NATS to the gateway 6.

The gateway 6 forwards this Alerting message through the BTS to themobile. The user now hears ringing tone.

When the called subscriber picks up a Connect message is sent by thePSTN through the MSC, gateway 9, GES, and SatCom/NATS to the gateway 6.

The gateway 6 forwards this Connect message through the BTS to themobile. The call is now established.

The gateway supports an external interface to the craft's In FlightEntertainment system or Public Address system. The gateway 6 recogniseswhen the IFE or PA system needs to make a voice broadcast to passengersand superimposes the crew broadcast on the call's audio signal. Thisallows the crew to interrupt calls for important messages.

In another embodiment, if the gateway 6 recognises that the recipient isregistered on the same craft the call may be switched locally in thegateway. This is to avoid using the craft-to-ground link bandwidthunnecessarily when both calling and called parties are logged onto thesame gateway 6. This local switching function can be enabled or disabledper class of subscriber.

Mobile Terminated Voice Call

The MSC determines the location area of the called mobile and sends apaging message to the gateway 9 (BSC). The gateway 9 respondsimmediately to the MSC and the MSC sends a set-up request. In this caseeach location area is associated with one or more movable gateways 6.The gateway 9 queries the database 32 to find the ocean region andconsequently the correct satellite to use for communications with theBTS.

The gateway 9 uses the set-up request from the MSC to build and send aset-up request to the mobile through the GES. This causes an ARINC 746Setup request to be generated in the SatCom/NATS module and sent to thegateway 6.

The gateway 6 uses the information in the Setup message to build aPaging Command and send it to the BTS.

The BTS pages the mobile on the Paging Channel (PCH). The mobileresponds with a Channel Required message on the Random Access channel(RACH).

The BTS forwards the Channel Required request to the gateway 6 whichresponds with a channel activation command to the BTS, the BTS activatesa signalling channel (SDCCH). The gateway 6 now sends an ImmediateAssignment Command to the BTS which is forwarded to the mobile on theAccess Grant Channel (AGCH). The mobile switches to the SDCCH and sendsa Paging Response through the BTS to the gateway 6.

The gateway 6 sends an ARINC 746 Call Proceeding message to the MSCthrough the satellite link, GES and gateway 9.

The gateway 6 sends a setup message through the BTS to the mobile whichresponds with a Call Confirmed message.

The gateway 6 now instructs the BTS to activate a traffic channel (TCH).The gateway 6 sends an Immediate Assignment command through the BTS tothe mobile which switches to the traffic channel. The gateway 6instructs the BTS to release the signalling channel (SDCCH).

The mobile starts ringing and sends an Alerting message to the gateway6.

When the subscriber picks up a Connect message is sent through the BTSto the gateway 6, the gateway 6 uses this message to build an ARINC 746Connect message which is sent to the SatCom/NATS module.

The gateway 9 relays this Connect to the MSC. The call is nowestablished.

Mobile Originated SMS

Once the mobile has registered successfully on the network it can sendan SMS message. The air interface used is standard GSM so the mobilesees no difference between the airborne BTS and a normal land BTS.

The mobile uses the Random Access Channel (RACH) to gain access to theBTS and request a signalling channel.

The BTS forwards the request to the gateway 6 which responds with achannel assignment command to the BTS. The BTS in turn instructs themobile to switch to the allocated channel (SDCCH).

Once the mobile has switched to the signalling channel, it sends an SMSrequest to the BTS which forwards the request to the gateway 6. Thegateway 6 sets up an X.25 circuit (if one is not already established)and uses this circuit to forward the request to the MSC.

The MSC forwards the SMS request to the subscriber's home SMSC whichhandles sending of the message to the recipient.

In another embodiment, if the gateway 6 recognises that the recipient isregistered on the same craft the SMS message may be switched locally inthe gateway. This is to avoid using the craft-to-ground link bandwidthunnecessarily when both the sender and the recipient are logged onto thesame gateway 6.

Mobile Terminated SMS

The MSC determines the location area of the called mobile and sends apaging message to the gateway 9 (BSC). In this case each location areawill have a single cell/BTS. The gateway 9 queries the Mobile GatewayDatabase to find the ocean region and consequently the correct satelliteto use for communications with the BTS.

The gateway 9 sets up an X.25 circuit to the airborne gateway (if one isnot already available) which is used to forward the paging message tothe BTS. The BTS pages the mobile.

When the mobile detects the page it will immediately request a channel.The gateway 6 allocates a signalling channel to the mobile which sends apaging response to the MSC on this channel.

The MSC now sends the SMS to the recipient mobile through the gateway 9,X.25 satellite link, gateway 6 and BTS respectively.

The gateway 6 can send SMS messages to any mobile station registered onthe craft. This facility is typically used to broadcast information suchas welcome messages to all registered users. These messages can be sentautomatically or when requested by the crew. Delivery of these internalSMS messages and intra-craft SMS messages do not require any craft-toground communication.

The existing conventional Pico BTS's available on the market todayconstitute practical small and economically viable elements for AirborneGSM solutions. However the BSC network element alternatives available onthe market do not meet the strict constraints of this particular marketsegment. If a product was to come to market that would satisfy such, theoverall network architecture of a ground based MSC communicating tothousands of individual BSS's would not be viable. There are severalsignificant technical constraints for such BTS only and BSS only, on anaircraft.

The Distributed BSC Architecture, consisting of ground based Gatewayseach supporting one or many on-board Movable Gateways, supports standardGSM A and A-bis interfaces. This allows the use of standardoff-the-shelf unmodified BTS and NSS components which significantlyreduces the cost of the system. Furthermore, the one-to-manyrelationship between Fixed Gateway and Movable Gateways minimizes thenumber of expensive signalling links needed in the NSS for a givenmobile population which reduces the cost even more.

Efficient use of satellite bandwidth is achieved through the Callhandling Model and Signalling Encoding Scheme.

The call model can make use of raw satellite bandwidth or can piggy-backon top of existing ISDN. (Q.931) based systems whichever is mostappropriate; this allows existing satellite equipment to be used in amost cost effective method. Data streams carrying voice traffic arecompressed to ensure optimal cost to quality ratios.

The signalling encoding and handling scheme ensures that the system usesthe least possible satellite bandwidth while supporting all necessarysignalling for compatibility with PLMN requirements such as mobileauthentication. All unnecessary signalling is terminated at the centralgateway 9 and remote gateway 6 so that only revenue-generating callrelated signalling and essential mobility management signalling occupysatellite bandwidth. In the absence of call traffic and mobilitymanagement traffic, no satellite bandwidth is being used.

In other embodiments, the BTS could support other radio protocolsincluding but not limited to the following:

-   -   CDMA (IS95) and evolutions of CDMA (CDMA 2000, CDMA 20001X etc)    -   UMTS and evolutions of UMTS    -   Evolutions of GSM such as GPRS, HSCSD, EDGE    -   TDMA    -   TD-SCDMA    -   Wireless LAN, RFC 802.11    -   Bluetooth

In another example the craft-to-ground communications systems could useother satellite or terrestrial based systems such as NATS, TFTS,Microwave, Intelsat, Globalstar, Iridium, ICO, Connexion by Boeing orother Inmarsat systems. Also, the remote parts may alternatively belocated in a fixed location, for example providing a small GSM facilityat a remote fixed location such as a remote village or regions generallywhere there is poor mobile network infrastructure. While the inventionas described splits a BSC into two gateways, it may instead split anequivalent mobile network node such as an RNC for a 3G network, or a PCUfor GPRS network.

In another aspect, specially customised mobile stations may be used atthe remote end. This customised mobile is intended for use in aircraftwhere there a risk or perceived risk of interfering with on-boardavionics equipment. This customised mobile station supports thefollowing features in addition to standard GSM features:

-   -   The mobile station has an external switch and indicator used to        put the mobile in “aircraft” mode. The switch position and        indicator are clearly visible externally so that they may be        checked by crew members.    -   When in “aircraft” mode, the mobile station will automatically        limit its output power to a pre-defined value, typically of the        order of 20 mW.    -   When in “aircraft” mode, the mobile station will only log on to        “aircraft” networks. The type of network (normal or “aircraft”)        is indicated in the Broadcast Control Channel (BCCH) of the        airborne BTS. For emergency calls, the mobile station may log        onto any BTS.    -   When in “aircraft” mode, the mobile station will accept Over The        Air (OTA) setting of ring and alert tones to silent mode using        SMS. This is to minimise disturbance to other passengers.

The BTS 5 and the gateway 6 are able to handle both conventional mobilestations and customised mobiles as described above.

The invention is not limited to the embodiments described but may bevaried in construction and detail.

1. A mobile network base station controller split into two physicallyseparated parts, said parts being a central gateway and a physicallyseparate remote gateway, in which: the remote gateway and the centralgateway together constitute a base station controller for a remote cellof the mobile network, the remote gateway being physically located inthe remote cell, the remote gateway communicates with cell equipmentincluding a base transceiver station in said remote cell via a standardprotocol for said remote mobile network cell, the central gatewaycommunicates via a standard protocol with a mobile network switchingnode, and the gateways perform internal base station controllercommunication between each other using a remote communication link suchthat the two gateways together constitute said base station controller;wherein the gateways use compression for at least some internalcommunication on the remote link.
 2. The mobile network base stationcontroller as claimed in claim 1, wherein the remote communication linkis a satellite link.
 3. The mobile network base station controller asclaimed in claim 1, wherein the remote communication link is a satellitelink; and wherein the remote communication link is a craft-to-groundcommunication link.
 4. The mobile network base station controller asclaimed in claim 1, wherein the remote communication link is terrestrialbased radio, microwave, cable or optical fibre.
 5. The mobile networkbase station controller as claimed in claim 1, wherein each gatewaybuffers speech packets and combines a plurality of buffered speechpackets into a single transmission packet.
 6. The mobile network basestation controller as claimed in claim 1, wherein each gateway buffersspeech packets and combines a plurality of buffered speech packets intoa single transmission packet; and wherein the transmission packet is anRTP packet.
 7. The mobile network base station controller as claimed inclaim 1, wherein the gateways perform transcoding and rate adaptation atthe most appropriate gateway according to the nature of the remote link.8. The mobile network base station controller as claimed in claim 1,wherein the central gateway comprises an operations and maintenancefunction for both gateways.
 9. The mobile network base stationcontroller as claimed in claim 1, wherein the remote gateway comprisesan operations and maintenance function for itself and the remote mobilenetwork cell.
 10. The mobile network base station controller as claimedin claim 1, wherein the central gateway comprises an operations andmaintenance function for both gateways; and wherein the operations andmaintenance function downloads updates to the remote gateway.
 11. Themobile network base station controller as claimed in claim 1, whereinthe central gateway comprises an operations and maintenance function forboth gateways; and wherein the operations and maintenance functionuploads fault information, status information and statistics from theremote gateway.
 12. The mobile network base station controller asclaimed in claim 1, wherein the central gateway maintains a database ofdata pertaining to remote gateways with which it communicates.
 13. Themobile network base station controller as claimed in claim 1, whereinthe remote gateway is linked to an entertainment or public announcementsystem.
 14. The mobile network base station controller as claimed inclaim 13, wherein the remote gateway allows calls to be interrupted bythe public announcement or entertainment system.
 15. The mobile networkbase station controller as claimed in claim 1, wherein the remotegateway determines if a locally-initiated call or message is to a personlocated in its cell, and avoids use of the remote link if this is thecase.
 16. The mobile network base station controller as claimed in claim1, wherein the central gateway detects Ocean Region changes from aremote gateway logon message and from the full address of the remotecraft, builds a full correct dialled number including the Ocean regionthat will be used to terminate voice calls to each subscriber.
 17. Themobile network base station controller as claimed in claim 1, whereinthe central gateway automatically generates a response to an MSC classmark enquiry.
 18. The mobile network cell comprising a base stationcontroller as claimed in claim 1, and a base station transceiver. 19.The method of communication between a remote mobile network cell and acentral mobile network node, the method comprising a base stationtransceiver of the remote cell communicating with a remote gateway of abase station controller of claim 1, the remote gateway processingsignals received from the transceiver and transmitting internal basestation controller signals to the central gateway via a remote link. 20.A mobile network base station controller split into two physicallyseparate parts, said parts being a central gateway and a physicallyseparate remote gateway, in which: the remote gateway and the centralgateway together constitute a base station controller for a remote cellof the mobile network, the remote gateway being physically located inthe remote cell, the remote gateway communicates with cell equipmentincluding a base transceiver station in said remote cell via a standardprotocol for said remote mobile network cell, the central gatewaycommunicates via a standard protocol with a mobile network switchingnode, the gateways perform internal base station controllercommunication between each other using a remote communication link suchthat the two gateways together constitute said base station controller;wherein the gateways perform transcoding and rate adaptation at the mostappropriate gateway according to the nature of the remote link.
 21. Amobile network base station controller split into two physicallyseparated parts, said parts being a central gateway and a physicallyseparate remote gateway, in which: the remote gateway and the centralgateway together constitute a base station controller for a remote cellof the mobile network, the remote gateway being physically located inthe remote cell, the remote gateway communicates with cell equipmentincluding a base transceiver station in said remote cell via a standardprotocol for said remote mobile network cell, the central gatewaycommunicates via a standard protocol with a mobile network switchingnode, and the gateways perform internal base station controllercommunication between each other using a remote communication link suchthat the two gateways together constitute said base station controller;wherein the gateways use compression for at least some internalcommunication on the remote link; wherein the remote gateway terminatesradio measurement signalling and link measurement signalling; andwherein the central gateway terminates link management signalling and“heartbeat” signalling from a mobile network node.
 22. A mobile networkbase station controller split into two physically separated parts, saidparts being a central gateway and a physically separate remote gateway,in which: the remote gateway and the central gateway together constitutea base station controller for a remote cell of the mobile network, theremote gateway being physically located in the remote cell, the remotegateway communicates with cell equipment including a base transceiverstation in said remote cell via a standard protocol for said remotemobile network cell, the central gateway communicates via a standardprotocol with a mobile network switching node, and the gateways performinternal base station controller communication between each other usinga remote communication link such that the two gateways togetherconstitute said base station controller; wherein the gateways usecompression for at least some internal communication on the remote link,and wherein at least one gateway terminates some signalling receivedfrom an external entity to minimize internal traffic on the remotecommunication link.
 23. The mobile network base station controller asclaimed in claim 22, wherein the remote gateway terminates “heartbeat”signalling from remote network nodes.
 24. The mobile network basestation controller as claimed in claim 22, wherein the remote gatewayterminates radio measurement signalling.
 25. The mobile network basestation controller as claimed in claim 22, wherein the remote gatewayterminates link measurement signalling.
 26. The mobile network basestation controller as claimed in claim 22, wherein the central gatewayterminates link management signalling.
 27. The mobile network basestation controller as claimed in claim 22, wherein the central gatewayterminates “heartbeat” signalling from a mobile network node.